Error message in find_data.default is not helpful

The error message in `find_data.default` gives no clue about where the error occurred or what the unknown data type is. To a user of a `lazyeval` client package it is quite mysterious and hard to debug. 

Something like
```
stop("lazyeval::find_data does not know how to find data associated with an object of class ", class(x), call. = FALSE)
```

would be more helpful.